Door restraining bars are old in the art but new improvements in methods and materials for defeating the bars demand a corresponding improvement to defend the bars from attack.
Restraining bars mounted horizontally across the inside of the door became vulnerable to saw blades and other cutting tools introduced through the interface between the door and the door frame. The first defense to this method of attack was the introduction of bars made of hardened metals which were difficult to cut with conventional saws and cutting tools. The introduction of saw blades and cutting tools having diamond cutting surfaces obsoleted the hardened metal bar.